Time-of-flight level measurement systems are used to determine the distance to a reflective surface (i.e. reflector) by measuring the time elapsed between the transmission of a signal towards a target and the return of the signal (i.e. echo) to the device. Time-of-flight level measurement systems may utilize ultrasonic pulses, pulse radar signals, or microwave energy signals.
Pulse radar and microwave-based level measurement systems are typically preferred in applications where the atmosphere in the container or vessel is subject to large temperature changes, high humidity, dust and other types of conditions which can affect signal propagation. To provide a sufficient receive response, a high gain antenna is typically used. High gain usually translates into a large antenna size.
Two types of antenna designs are typically found in radar-based and microwave-based level measurement systems: rod antennas and horn antennas. Rod antennas have a narrow and elongated configuration and are suitable for containers having small opening/flange sizes and sufficient height for accommodating the antenna. Horn antennas, on the other hand, are wider and shorter than rod antennas. Horn antennas are typically used in installations with space limitations, for example, vessels or containers which are shallow.
The level measurement instrument or device comprises a housing and an waveguide such as an antenna. The level measurement device is mounted on top of a container or vessel containing a material with the antenna extending into the interior of the vessel. The level measurement instrument is typically bolted to a flange around an opening in the top of the container. The instrument housing holds the electronic circuitry for transmitting and receiving the signal, calculating the time elapsed between transmission and reception of the signal, and determining the distance to the surface of the material. The antenna is coupled to a transducer in the instrument housing. When the transducer is excited by the control circuitry, the antenna transmits electromagnetic energy pulses into the vessel, and receives pulses that are reflected back from the surface of the material contained in the vessel.
The material contained in the vessel often sticks to horn antennas of level measurement devices. A build-up of material on the inside of a horn antenna prevents efficient signal transmission and also causes “noise” in received signals. This build-up of material can cause false measurements or signal loss. If the materials adhered to the inner surface of the horn antenna are removed by cleaning, normal signal transmission and reception is returned and proper measurement can again be obtained from the level measurement device.
One method of cleaning is to manually clean the horn antenna. This involves removing the level measurement device from service and cleaning it with brushes, scrapers, or the like, and then re-installing the cleaned device. This is time consuming, costly, and sometimes unsafe. Another option used for periodic cleaning is air or liquid purging. This involves connecting an air or liquid supply to the top side of the horn antenna (for example, through an inlet on the mounting flange) and activating the purging cycle manually or automatically, for example using a solenoid valve. The air or liquid passes through to the inside of the horn antenna and flushes it out. While effective, this technique requires air or water to be supplied to the level measurement device which can be problematic in some applications. Air and/or water supply is usually not available at the top of the container. Thus, an air or water line must be installed which is costly to implement. Typically, compressed air is used for purging because most solid applications cannot tolerate water inside the vessel. Compressed air, however, is relatively costly to produce for continuous process requirements.
Accordingly, there remains a need for a cleaning mechanism for level measurement systems, in particular radar-based and microwave-based, which addresses these deficiencies.